Boreholes, which are also commonly referred to as “wellbores” and “drill holes,” are created for a variety of purposes, including exploratory drilling for locating underground deposits of different natural resources, mining operations for extracting such deposits, and construction projects for installing underground utilities. A common misconception is that all boreholes are vertically aligned with the drilling rig; however, many applications require the drilling of boreholes with vertically deviated and horizontal geometries. A well-known technique employed for drilling horizontal, vertically deviated, and other complex boreholes is directional drilling. Directional drilling is generally typified as a process of boring a hole which is characterized in that at least a portion of the course of the bore hole in the earth is in a direction other than strictly vertical—i.e., the axes make an angle with a vertical plane (known as “vertical deviation”), and are directed in an azimuth plane.
Conventional directional boring techniques traditionally operate from a boring device that pushes or steers a series of connected drill pipes with a directable drill bit at the distal end thereof to achieve the borehole geometry. In the exploration and recovery of subsurface hydrocarbon deposits, such as petroleum and natural gas, the directional borehole is typically drilled with a rotatable drill bit that is attached to one end of a bottom hole assembly or “BHA.” A steerable BHA can include, for example, a positive displacement motor (PDM) or “mud motor,” drill collars, reamers, shocks, and underreaming tools to enlarge the wellbore. A stabilizer may be attached to the BHA to control the bending of the BHA to direct the bit in the desired direction (inclination and azimuth). The BHA, in turn, is attached to the bottom of a tubing assembly, often comprising jointed pipe or relatively flexible “spoolable” tubing, also known as “coiled tubing.” This directional drilling system—i.e., the operatively interconnected tubing, drill bit and BHA—is usually referred to as a “drill string.” When jointed pipe is utilized in the drill string, the drill bit can be rotated by rotating the jointed pipe from the surface, through the operation of the mud motor contained in the BHA, or both. In contrast, drill strings which employ coiled tubing generally rotate the drill bit via the mud motor in the BHA.
Many conventional drilling motors include a progressive cavity, positive displacement motor (PDM) to provide additional power to the bit during a drilling operation. As an alternative to PDMs, some BHAs will employ a turbine-based motor (or “turbodrill”) to provide the additional power. Both PDM and turbine motors are fluidly driven by the drilling mud pumped down the drill string, through the drilling motor, and out the bit assembly. After exiting the distal end of the drill string through ports in the drill bit, the drilling fluid operates, in part, to carry drill cuttings from the drill bit to the surface up through the annulus between the drill string and the wall of the borehole. Conventional PDMs typically operate at a slow rotational velocity with a high torque output; contrastingly, turbines typically operate at high rotational velocities with a low output torque.
Historically, it was desired to carry out drilling operations in a low speed, high torque operation to reduce the likelihood of the drill bit sticking in the formation and, thus, the likelihood of damage to the BHA in the event that the drill bit does become stuck. For this reason, PDMs, which typically operate at slow speeds and generate high torque, tend to be the predominant workhorse in borehole drilling. PDMs, however, include some components that can be damaged under the high pressures and temperatures experienced during a drilling operation. Damage to these components can lead to failure of the PDM which, in turn, requires costly, time-consuming replacement. To reduce downtime and repair costs, it is sometimes preferred to use turbine-based drilling motors which do not normally include such easily damaged components. However, as noted above, turbines are high speed, low output torque motors; thus, it is often required to provide a speed-reduction mechanism to reduce the rotational velocity of the turbine.
Some drilling operations now require a high-speed, high-torque output mud motor. With recent developments in drilling technology, including improvements in lubrication capabilities and the availability of high-performance drill bits, a number of complex-bore drilling operations can be performed at high rotational velocities and with high torque. Other operations which can benefit from a high-speed, high torque output motor include, for example, drilling vertical boreholes, drilling in soft formations, and directional applications where single-shot orientations are being used. However, conventional PDM assemblies and turbine-driven mud motors do not provide both high-speed and high-torque output functionality.